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Algae (singular alga) is a term that involves many different groups of living organisms. Algae capture light energy through photosynthesis and convert inorganic substances into simple sugars using the captured energy. Algae range from single-celled organisms to multi-cellular organisms, some with fairly complex differentiated form. Algae have been traditionally regarded as simple plants, and some are closely related to the higher plants. The absence of vascular system in algae differentiates them from higher plants. Reproduction in algae exhibits over a wide range from simple vegetative, asexual cell division to complex forms of sexual reproduction.

The three most prominent groups of algae are the brown algae, the red algae, and the green algae, amongst which some of the most complex forms belong to green algae. This lineage eventually led to the higher land plants. Algae are some of the most robust organisms on earth and are able to grow in a wide range of conditions. They are usually found in damp places or water bodies and thus are common in terrestrial as well as aquatic environments. However, terrestrial algae are usually rather inconspicuous and far more common in moist, tropical regions than dry ones, because algae lack vascular tissues and other adoptions to live on land. An alga forms the lowest trophic level of food pyramids operating in aquatic systems.

Algae grow in almost every habitat in every part of the world, on animals like turtles, snails, rotifers, worms, crustacea and many other animals;, on aquatic plants and inside water plants (including other algae), on artificial substrates as on wooden posts and fences, cans and bottles. Some more algae habitats are billabongs & lagoons, bogs, marshes & swamps, farms, dams, hot springs, lakes, mud and sand, ponds, puddles, roadside ditches and rock pools, water reservoirs, rivers, rock (internal & surface), saline lagoons, saline lakes & marshes, snow, soil, water streams etc.

Periodicity of algae: The growth of algae is highly depended upon seasonal variations and fluctuation in temperatures, salt concentration, light intensity, dissolved gases, humidity etc ., but the interaction of these various factors is so complicated that it is difficult to figure out role and action of each factor in the growth of algae.

Classification of Algae

F E Fritsch proposed 11 classes of algae which are as follows:

Chlorophyceae (Green algae)

Cryptophyceae

Phaeophyceae (Brown algae)

Rhodophyceae (Red algae)

Xanthophyceae (Yellow-green algae)

Dinophyceae

Bacillariophyceae (Diatoms)

Chloromonadineae

Eugleniae

Chrysophyceae

Myxophyceae.

Algae with their increasing popularity due to their versatility are finding applications in many emerging spheres as nutritional supplement, fertilizer, pigment and stabilizing agents, neutraceutical, pharmaceutical and in medical fields.

Algae have been recognized as a rich energy source and thus have been utilized to produce many biofuels such as hydrogen, methane, bioethanol, biodiesel and many more. These renewable fuels produced are being termed as third generation fuels.